1. Field of the Invention
The present invention relates to a local oscillator.
2. Description of the Related Art
In an all digital phase locked loop (ADPLL) using an oscillator (a digitally controlled oscillator (DCO)) that can discretely control an oscillation frequency, a quantization error due to discretization appears in an output of the oscillator as phase noise. Therefore, in the oscillator used in the ADPLL, it is important to reduce the quantization error, i.e., reduce a variable amount of a frequency for each of control signals (control codes) that assume discrete values.
The present invention intends to realize an oscillator (DCO) used in a low-frequency ADPLL taking into account the requirements explained above and to configure the ADPLL using a ring oscillator that can be reduced in size.
A general ring oscillator controls an electric current flowing through delay circuits to change a delay amount and control an oscillation frequency. A discrete change in an oscillation frequency in such a ring oscillator can be realized by switching a current ratio in a current mirror. However, in general, a frequency variable amount of the ring oscillator with respect to an electric current is extremely large. To realize a seemingly practical frequency interval, it is necessary to control the electric current at an extremely small interval. It is likely that, if a current value is switched, noise is superimposed on a power supply to adversely affect an oscillation frequency.
For example, Japanese Patent Application Laid-open No. 2008-54134 proposes, as a method of changing a delay amount without controlling an electric current flowing through delay circuits, a method of providing variable capacitance circuits in parallel to resistor elements respectively provided between output terminals of differential amplifier circuits and a circuit ground in a ring oscillator, which includes a differential amplifier circuit, i.e., a differential non-inverting delay circuit as a delay circuit, and changing capacitance values of the respective variable capacitance circuits according to bit values of control signals of a plurality of bits. However, in this method, it is difficult to distribute uniform capacitance to the output terminals and it is impossible to secure linearity of an oscillation frequency.